I used to have an adapter with a male 3.5mm plug, and mounted directly on the back of it, with no leads, was an SO-239 female coax connector. I used that with a Sony 2010 back in the 90s. The stiffness of the coax cable may have put a strain on the radio's antenna jack, but it survived. I've misplaced the adapter since then, but I'm pretty sure I bought it at Radio Shack. Maybe they still carry it.
My ATS-909X has four 3.5mm jacks (antenna, auxiliary input, line out and headphones). They're all on the left side panel, so I often connect things to the wrong jack by mistake. Kind of frustrating, because sometimes I wonder why I can't hear anything in the headphones.